Tapered stem geometry provides superior initial fixation stability to cylindrical stem geometry in the setting of severe bone loss: A finite element analysis

Abstract

Initial stability of cementless stems used in total hip arthroplasty (THA) is critical for subsequent osseointegration at the bone/implant interface. In the setting of revision THA, there is frequently less intact femoral bone available for fixation of the new stem. This study aims to compare the initial fixation stability between a cylindrical and tapered stem, designed for diaphyseal fixation in a revision THA setting, with increasing bone defect severity. Using finite element analysis, severe Paprosky Type III femoral bone defects were simulated. The cylindrical stem had a 13.3% higher construct stiffness as compared to the tapered stem for the least severe bone defect modeled. In contrast, for the most severe bone defect, the tapered stem showed a 12.3% higher construct stiffness than the cylindrical stem. At the bone/implant interface, the tapered design resulted in less than 20 μm of maximum tangential micromotion for all the bone defect models, whereas the cylindrical stem exceeded 20 μm for all and was as high as 70 μm in the most severe case. Given the limitations of the models presented, the results suggest the tapered stem obtained superior initial fixation as compared to cylindrical stem with increasing bone defect severity.

Micromotion and Stress Shielding Between Taper Fluted and Cylindrical Femoral Stems for Paprosky Bone Deficiency Type IIIB

“…By reducing the modulus mismatch between the femoral component and the host bone, better load transfer to the surrounding bone is expected. In addition, finite element analysis reported smaller micromotion at prosthesis -bone interface in tapered stem [8]. Furthermore, the incidence of intraoperative fracture and postoperative thigh pain was lower in the tapered design [9].…”

“…Although both cylindrical and tapered fluted distal fixation stem can be used in a proximal femoral bone loss reconstruction with a good success rate [8,10,11], the difference between the two stem designs for reconstruction especially in type IIIB femoral bone loss is not well demonstrated. Paproky and Sporer [12] mentioned an increase in failure of fully porous-coated cylindrical stems in a large femoral canal diameter >18mm, which is often seen in type IIIB and IV femoral bone defects.…”